The present invention relates to ultrathin, porous, and at the same time mechanically stable nonwoven fabrics, their manufacture, and their use as, for example, separators for electrochemical cells such as accumulators, batteries, or fuel cells, as well as for electrical energy storage such as super-capacitors.
Electrochemical cells must have separators which separate the two differently charged electrodes within the cell, thereby preventing an internal short-circuit. A series of demands are made on separator materials which may be summarized as follows:                1. Resistance to the electrolyte        2. Resistance to oxidation        3. High mechanical stability        4. Low weight tolerance and thickness tolerance        5. Low ion passage resistance        6. High electron passage resistance        7. Ability to retain solid particles detached from the electrodes        8. Instant spontaneous wettability by the electrolyte        9. Permanent wettability by the electrolyte, and        10. High storage capacity for the electrolyte fluid.        
Textile fabrics, in particular nonwoven fabrics made of synthetic fibers, are inherently well suited to be used as separator materials because of their good resistance to electrolyte fluids and at the same time their high flexibility.
Current material alternatives are papers which, however, have only a low porosity due to their high density and are therefore unsuitable for applications which require an open material. Other material alternatives are diaphragms which, however, are limited with respect to the polymers to be used and have, as a rule, only a low porosity of <25% and small pore diameters of <0.5 μm. So far, a thickness of <20 μm could not be achieved in the nonwoven products known per se. Nonwoven products having substantially reduced thicknesses and at the same time excellent mechanical properties as well as high porosity are desirable for many novel applications.
Such nonwoven fabrics may be used, for example, as separator support materials in lithium batteries, alkaline batteries, super-capacitors, or fuel cells, as well as as a carrier material for filter diaphragms.
The use of nonwoven fabrics as a carrier material or a support material for diaphragms is known. Fluid is pressed and filtered under high mechanical pressures through a diaphragm reinforced in this way.
The following demands are made on this support material:                Chemical resistance to the solution to be filtered        Sufficient mechanical stability        “Compatibility,” i.e., good adhesive properties of the base material with the diaphragm polymer which is applied for the most part using the extrusion method        Extremely smooth surface so that no projecting fiber is able to perforate the diaphragm.        
The properties for these supported materials are weighted as follows:
stability>>homogeneity=thickness>structure
The mechanical stability of the support materials is of utmost importance. Typical values for the maximum tensile load are in the range of >>200 N/5 cm. This yields thicknesses and masses per unit area of >200 μm and >60 g/m2, respectively, when conventional materials are used.
Since diaphragms used in such systems have very small pore diameters (as a rule <<1 μm) and low porosity, the presence of high porosity and a uniform pore-size distribution in the support material is secondary. Therefore, coarse fibers (titer >1.5 dtex) are used as a rule in such materials. The presence of a labyrinth-like structure and a small thickness in these materials is not absolutely essential for this application.
If nonwoven fabrics are to be used as carrier materials for separators, for example, in lithium batteries, alkaline batteries, and fuel cells, they must meet the criteria mentioned above.
The chemical properties are to be selected in such a way that there is    a) chemical resistance to the electrolyte (organic media in the case of Li cells, highly acidic aqueous solutions in the case of fuel cells, highly alkaline solutions in the case of alkaline batteries) at long-term temperatures typically of up to 70° C. and resistance to oxidation, and that    b) no or at least only minor mechanical swelling of the separator takes place in the event of contact with the electrolyte.
In addition, the properties which are determined by the morphology/geometry of the material are to be selected in such a way that there is/are:    a) a small thickness of <30 μm, preferably <20 μm (the ion passage resistance of the separator is a linear function of the thickness);    b) high porosity and at the same time homogeneous pore distribution (the porosity directly affects the ion passage resistance; the porosity of such a material should therefore amount to >25%, preferably >45%, and, to ensure a uniform pore distribution, the maximum pore size should be at the most 2.5 times that of the most likely pore size);    c) a small pore size, so that dendrites in batteries cannot become intermixed (however, when nonwoven fabric is used as a support material, this issue plays a subordinate role, since, as a rule, the gel/compound introduced assumes this task; for both applications, however, a maximum pore size of typically 500 μm should be observed);    d) sufficiently large pore sizes which make the introduction of a compound or gel possible (the minimum mean pore sizes should not exceed 0.5 μm);    e) a sufficiently high mechanical stability for the assembly of the cells (in practice, maximum tensile load values of at least 15 N/5 cm have been found to be suitable for industrial processing);    f) minor stretching of the material when mechanical stress is applied (excessive stretching values may result in material distortion); it has been shown in practice that stretching values under maximum tensile load should not exceed 35%; and that    g) simple manufacture of the separator is possible.
In applications other than the above-mentioned applications as a carrier material or as a support material for diaphragms, the weighting of the properties is as follows:
thickness>>porosity=homogeneity=structure=mechanical stability
It is generally possible to achieve nonwoven fabrics having thicknesses of <30 μm by calendering materials of different types. It should be pointed out, however, that a polyolefin fiber having a fiber titer of 2 dtex, which is used as a standard material in separator applications, already has a fiber diameter of approximately 17 μm itself, which means that a 50 μm thick material may only be composed of a maximum of three layers of such a fiber. Calendering of a comparatively heavy material, with respect to the mass per unit area, may yield a thin material, the resulting material, however, being so dense in this case that it would result in a high ion passage resistance.
Therefore, if sufficiently high mechanical stability, high homogeneity, as well as narrow pore distribution are required in addition to the small thickness, the use of meshes (materials are too coarse-fibered and the pores thus too large) and the use of papers (materials are too dense) are preferred to a lesser extent.
The separators in Li-ion accumulators are, as a rule, microporous diaphragms, mostly made of polyolefins. The porosities are comparatively low and are in the region of <25%. Hence, the resulting electric resistance is high. They do not have any explicit ionic conductivity. Microporous polyolefin diaphragms which have been laminated at least on one surface using a nonwoven fabric made of polyolefin fibers and their use as separators are described in European Patent Application No. 0 811 479. According to the description, the polyolefin nonwoven has a thickness of 30 μm to 500 μm prior to laminating.
U.S. Pat. No. 5,500,167 describes a microporous diaphragm having a carrier for the filtration. A porous nonwoven fabric is used as a carrier. No details can be found about the thickness of the nonwoven fabrics used; however, fibers having a diameter of 20 μm to 25 μm are used for their manufacture. Thus, the resulting nonwoven fabrics have thicknesses which are considerably above 50 μm.
A diaphragm element for reversible osmosis, made of multiple layers, is known from U.S. Pat. No. 6,277,282. One of these layers may be made of a nonwoven fabric. According to the description, this layer has a thickness of 50 μm to 200 μm. Diaphragms made of ion-conductive polymers are presently used for Li-polymer accumulators (as described, for example, in DE-A-199 16 109; DE-A-199 16 043; DE-A-198 55 889, and European Patent Application No. 0 662,250 corresponding to WO-A-93/13, 565). During their manufacture, the components, dissolved or dispersed in an organic solvent, are applied to a film and the solvent is evaporated in a defined manner. This is followed in most cases by a thermal or UV-induced crosslinking process. The ion-conductive diaphragm manufactured in this way has a substantially lower resistance than the microporous diaphragms described above. As a rule, these diaphragms made of ion-conductive polymers are laminated onto the electrodes in a continuous step. The low mechanical stability of such thin diaphragms is particularly problematic. They may develop cracks or break completely during manufacturing. Irreparable damage to the future cell occurs in the first case; production must be stopped in the second case. A remedy is found in practice in that such diaphragms are initially deposited on a film which is subsequently removed. As a rule, the film is not reused. In addition to the waste material and the associated extra costs, problems may arise during the diaphragm's removal from the film; in this case also, breakage of the diaphragm cannot be ruled out.
A button cell battery is known from JP-A-2000-195,494 in which, among other things, a nonwoven separator may also be used. In addition to thermostable structure materials, this nonwoven fabric is made of a polymer which expands in contact with the electrolyte and absorbs it. No details are given about the thickness of these nonwoven fabrics.
JP-A-11/176,419 describes a secondary lithium cell which is made of a multi-layer electrode-separator system. The separator having a thickness of 20 μm to 200 μm is made of thermostable structure materials, in this case polyvinylidene fluoride (PVDF) or PVDF-HFP (hexafluoropropylene) mixtures. This document describes a good behavior at nonwoven fabric thicknesses between 50 μm and 100 μm. It was not possible to achieve thicknesses below 20 μm due to the nonwoven fabric's low breaking resistance.
WO-A-00/77,875 describes the simplification of the manufacturing process of a secondary lithium-polymer cell which is made of a multi-layer electrode-separator system. According to this document, thin electrode materials are fabricated which are deposited from an organic solution on a film made of polyolefin or polyester, on paper, or on a very heavily calendered, dense polyamide nonwoven. This “deposit” is used as a processing aid for the subsequent laminating (“strip casting”) of the electrodes using binders. Details about geometrical dimensions of the deposit are not to be found in this document.
WO-A-99/31,743 describes the deposition of dispersed electrode layers on a separator surface. Details about geometrical dimensions of the deposit are not to be found in this document either.
Thin nonwoven fabrics and methods for their manufacture by separating bulky and bonded nonwovens into thin layers are described in DE-A-25 47 958. Nonwoven fabrics having masses per unit area down to below 20 g/m2 are described. Although details about the thickness and the mechanical properties of these nonwoven fabrics are not to be found in this document, it concerns, however, a bulky and open starting material. The nonwoven fabrics made of that material thus do not have particularly good mechanical properties.